The Sarasota Bay Listening Network reveals sounds from boats, fish, and other marine creatures. But it’s not all noise. Many sounds convey information that is important to the marine community. Fish probably first developed the ability to hear sounds in order to sense their surroundings. Developing the ability to make sounds likely came much later.

A new exhibit at the Mote Aquarium, shown by this photo, lets visitors hear the sounds of many different fish. Some are named for the sounds they make, like the Blue Striped Grunt and Black Drum. You can hear recordings of many common fish sounds here.

How do fish hear?

Fish have two different ways of detecting sounds. Like all vertebrates, they have inner ears. But they have no external ears. That is because the density of a fish’s body is almost the same as water, so sounds pass directly through its body to its inner ears. A fish’s hearing is affected by its anatomy, especially the location of the inner ear relative to its air-filled swim bladder.

How well a fish hears depends on how important hearing is to its survival. Fish that stay mostly in the same place don’t need super sensitive hearing. The Gulf Toadfish, an ambush predator, spends most of its time blending in with its surroundings. But the toadfish mating call attracts dolphins, so a toadfish hears well enough to stop its mating call when it detects a nearby clicking sound from a dolphin.

By contrast, fish that are always on the move, like shad, can detect sounds at frequencies well up into the ultrasonic range, 180 kHz. Dolphins use this frequency for echolocation. The American shad is one of the few fish able to hear such high frequencies, a result of specially-evolved inner ear capabilities.

A second kind of hearing system is found only in fish. This is the lateral line, extending from head to tail on both sides of a fish’s body. It is very visible in some fish, like this snook. The lateral line is an organ that detects movement, vibration, and pressure in the surrounding water. These may come from a nearby fish in a school, from prey, or from a predator. Fish swimming in a school depend on their lateral lines to maintain spacing and to synchronize speed and direction.

How do fish make sounds?

Fish make some sounds by swimming or feeding. But they also make sounds through mechanisms designed for that purpose. While the main purpose of the swim bladder is to regulate buoyancy, some fish make a drumming sound from vibrating muscles next to the swim bladder. Frequency varies from low (45-60 Hz for black drum), to intermediate (250-300 Hz for toadfish) to high (1000 Hz for silver perch).

Other fish make sounds by rubbing their teeth or hard parts of the skeleton together. These sounds can have much higher frequencies than those coming from the swim bladder, over 8 kHz. Marine catfish and seahorses make sounds this way. Grunts (the fish) reinforce rubbing sounds by using the swim bladder as a resonator. Anatomical differences between fish species cause corresponding differences in their sounds. More detail on how fish make sounds, and example recordings, are here.

Sarasota Bay Listening Network

The Sarasota Bay Listening Network (SBNL) consists of hydrophones and recorders located throughout Sarasota Bay. They automatically transmit their data via cell phone modems. This Google Earth image shows current station locations, one of which is on the North side of Tidy Island. New College of Florida, the Sarasota Dolphin Research Program (SDRP), and Loggerhead Instruments manage the project. The Chicago Zoolological Society supports the SDRP from facilities at the Mote Marine Laboratory in Sarasota.

Sarasota Bay Listening Network and the 2018 red tide

The algae Karenia brevis causes red tides when its concentration increases to about about one-million cells per liter. At that concentration, toxins produced by the algae can kill fish and other marine life. SBLN installed its first two listening stations (circled on the Google Earth Image) in 2018. Accordingly, those stations were able to detect the rapid onset of a severe red tide in August. The sudden increase in K. brevis concentration happened at exactly the same time as a sudden decrease in fish sounds. Athena Rycyck and her collaborators from SDRP and Loggerhead Instruments published these observations in Scientific Reports. Athena is a New College marine mammalogist who is especially interested in manatees. She has visited Tidy Island many times and is responsible for the listening station here.

Data from the listening stations in the Scientific Reports article extended through mid-October of 2018 but the effects of the red tide lingered through January of the next year. As a result, the beginning of fish population recovery, signaled by the return of sounds associated with silver perch, did not occur until much later in 2019.

Dolphins have a lot to say

The SDRP has identified nearly 250,000 individual dolphin whistles. Three particular whistles were heard repeatedly at the furthest north station in Sarasota Bay. Those whistles were connected to a mother and her two calves. Now they announce their presence whenever they visit a listening station. Here is a good explanation of how dolphins create sounds, including recordings of dolphin whistles and clicks.

The SDRP studies involve more than just dolphin sounds. It is a 50-plus-year study of the resident dolphin population in an around Sarasota Bay. It covers five dolphin generations and includes individual dolphins as old as 66 years. Tagging and tracking began with Mote Marine Laboratory in 1970. The research partnership with Chicago Zoological Society began in 1989 and has been based at Mote Marine Laboratory since 1992. The work includes photographic identification of individual dolphins based on dorsal fin nicks and notches. Almost all Sarasota Bay dolphins are visually identifiable. More information is here.

Capture-release of dolphins complements photo identification, allows radio-tagging, and enables health assessments. This photo shows a dolphin capture-release off the south shore of Tidy Island. The Mote volunteers in the water have corralled the dolphin who accepts the gentle handing required for its physical examination and health check without any fuss.

Manatees not so much

In contrast to dolphins, manatees (and manatee sounds) have not been studied very much. They don’t seem to make individual sounds like dolphin whistles. Nor do they use sound for navigating or locating food. But they make special sounds when annoyed or frightened. As do mothers and calves to find one another. Nursing calves also have a special call. Here are some examples.